ABSTRACT Youth between the ages 13 and 24 are disproportionately represented in the United States HIV epidemic; in 2015, 22% of HIV infections occurred within this age group. Youth living with HIV (YLH) face significant barriers to care engagement across all stages of the HIV Care Continuum. The transition from pediatric/adolescent to adult-oriented care settings can be especially disruptive. Yet, there are no evidence- based interventions shown to improve healthcare transition (HCT) clinical outcomes for YLH. We propose to develop and pilot iTransition, a mobile website-based intervention for YLH and their providers to improve HCT at the patient, provider, and clinic levels. To maximize the cultural and developmental congruence of the intervention we will convene a Design Team that includes YLH and healthcare providers, who will inform the development of the intervention content and delivery strategies. Using an iterative design and development process, we will develop tailored content and activities that align with YLH-specific HCT needs. After developing the intervention, we will employ established usability testing procedures to collect preliminary data from our Design Team on the proposed intervention's design and acceptability. Providers on the Design Team will also be trained as ?Transition Champions, encouraging and facilitating uptake of the intervention in their respective clinics. Recommended changes will be added to the intervention prior to launching an implementation trial based at pediatric/adolescent and adult-oriented HIV clinical care sites in Atlanta and Philadelphia. The trial will use a mixed-methods approach to assess intervention feasibility and acceptability for YLH (N=50) and provider (N=20) participants. We will also compare clinical outcomes of YLH in the iTransition intervention group (N=50) over the 12-month period around HCT, to outcomes of a historical comparison group (N=50), in order to gather preliminary efficacy data to inform a subsequent application. Specific Aims include: (1) To develop iTransition, an mHealth intervention designed to improve care engagement throughout HCT by targeting YLH, pediatric/adolescent providers' and adult providers' needs; and (2) To conduct a pilot trial of iTransition with 50 YLH and 20 providers, in order to collect data on feasibility, acceptability, and preliminary efficacy for improving HCT. Future directions include a fully powered RCT to test the efficacy of iTransition for improving post-HCT care continuum outcomes for YLH.